Alkyl halides are commonly manufactured by hydrohalogenation of alcohols. Thus, methanol and hydrogen chloride are contacted over a suitable catalyst to make methyl chloride. The hydrochlorination reaction produces an equimolar amount of water along with the desired alkyl chloride. Before the alkyl halides can be employed for most uses it is necessary to dry them. For many uses the water content must be reduced to 0.1 percent or less, even to the low parts-per-million range.
A common way of drying methyl chloride is to contact the effluent gas stream (which has had most of the water removed) with concentrated sulfuric acid. This method results in a dry methyl chloride, but the gas stream carries over some acid which is corrosive to equipment and undesirable in many reactions in which the methyl chloride is employed.
A more recent development in drying methyl chloride is described in U.S. Pat. No. 4,145,260, wherein a distillation of wet methyl chloride is conducted in the presence of hydrogen chloride to produce aqueous HCl bottoms and dry methyl chloride overhead.
In related art nitrogen trichloride and water were removed from chlorine gas by counter-current contact with aqueous hydrochloric acid (26% HCl) at 0.degree. C. This process is described in U.S. Pat. No. 3,568,409.
Thus, it would be desirable to have a method for drying methyl chloride which would avoid the use of sulfuric acid as a drying agent.
It has now been discovered that a more convenient method of drying is to cool the gas to a temperature which condenses out substantially all of the water without condensing the methyl chloride itself.